Coupled circuit regenerative receiving system



July 22, Q. W A, SCHAPER 2,250,277

GOUPLED CIRCUIT REGENERATIVE RECEIVING SYSTEM Filed Aug. 2, 1940 2Sheets-Sheet l awww@ ATTORNEY July 22, 1941. w. A. sczHAF'ER'` COUPLEDCIRCUIT REGENERATIVE RECEIVING SYSTEM Filed Aug. 2, 1940 '2 sheets-sheet2 INVENTOR Mum/w ,4. 6CH/4PM ATTORNEY Patented July 22 1941 naar COUPLEDCIRCUIT REGENERATIVE RECEIVING SYSTEM William A. Schaper, Cicero, Ill.,assignor to'Jolinson Laboratories, Inc.,

tion of Illinois 9 Claims.

f The utility of a selective resonant high-fre quency system isdetermined principally by (1) its resonant performance, 4(2) the rangeof frequencies over which it may be adjusted, (3) the ease with which itmay be adjusted to any frequency within its range, (4) the uniformity ofits resonant performance over the frequency range, (5) the degree ofdifficulty encountered in ganging it with similar systems forsimultaneous adjustment, and (6) its cost.

The resonant performance of the system determines the increase in anyimpressed voltage to which the system is adjusted, which iscommonlycalled the resonant gain, and it also determines the degree towhich the system discriminates against voltages of undesiredfrequencies, which is commonly called the selectivity. It is apparentthat, within limits, the better the resonant performance the greater theutility of the system, provided the improvement is not secured atdisproportionate increase in cost.

Although selective resonant systems have been employed in radioapparatus for many years, it has only recently become possible to designsystems which have substantially constant performance over the range ofadjustability, and at the same time have relatively high eiiiciency.Such a system is the one disclosed by Polydoroif in United States PatentNo. Re. 21,282, in which a resonant circuit having an inductance coiland a capacitor is adjusted over a range of frequencies by movement of acompressed comminuted ferromagnetic core relative to the inductancecoil. This method of tuning is commonly called permeability tuning. Animproved form of such a system is described in my United States PatentNo. 2,051,012. Both Polydoroffs original system and my improved systemreadily cover an adequate range of frequencies and may easily be gangedto provide multiple unit systems.

' With regard to the matter of resonant performance, however, taken inrelation to cost, it has been found that in order to secure adequategain and selectivity, for example in a. broadcast Chicago, Ill., acorpora- Application August 2, 1940, Serial No. 350,052.

receiver, it is necessary to employ several such systems gangedtogether, andy electrically connected in'the well-known cascadearrangement maintaining accurate alignment when several such systems areemployed, such arrangements are not widely used in spite of the greatadvantage which they possessof uniformity of performance over a range offrequencies.

Further improvement of the systems themselves to secure an increase ingain and selectivity that would materially decrease the number ofsystems required in a complete receiver has thus far not been possible.in the eiiiciency of the low-loss inductance coils could be secured onlyat greatly increased cost, not only of the coils themselves but also ofthe ferromagnetic cores which had to be employed with them.

It is to the solution of the problem above suggested that my presentinvention is addressed.

. In accordance with my present invention I employ coupled resonantcircuits tuned by inductance variation with movable ferromagnetic coresand associated with amplifying vacuum tubes, but I additionally employmeans for very greatly increasing the resonant performance of suchcircuits, so that even though they may be, and preferably are,constructed with relatively inefficient coils, two or at thev most threepairs of such circuits arranged in accordance with my invention provideadequate gain andselectivity, for example in a broadcast receiver.

The means which I employ to increase the resonant performance of myamplifier is, in some respects, broadly similar to sci-calledregenerative arrangements to be found in many forms of apparatus inwhich oscillations are produced. It is necessary to point out,therefore, that the arrangement here described differs in importantstructural aspects and in the manner of its operation frorn any earlierarrangement which diligent search in the literature of the art hasrevealed. Additionally it is pointed out that in arrangements inaccordance with my invention no oscillations are produced.

The use of negative resistance or regeneration to improve theperformance of selective resonant systems was common in the early daysof the receiver art, but was abandoned over ten years ago, at least sofar as broadcast receivers in the Even' slight improvements the negativeresistance or regenerative effect.

The demand for increased simplicity of control, and finally foruni-control, drove the regenerative receivers off the domestic market.

Many attempts were made in the period from 1921 to 1926 to provide asystem of regeneration which would be'uniform in action over the rangeof frequencies. due principally to lthe non-uniform performance of thecircuits then employed, with the resultkk that the art turned tocompletely. non-regenerative-types such as to-so-called Neutrodynereceivers, and'to the superheterodyne method in which the problem ofhigh-frequency selectivity and gain was avoided by transposing all thesignais to a fixed lower frequency.

It might be lsupposed that immediately upon the advent of' theuniform-performance permeability-tuned circuit, it.would have vbeenpossible to add regeneration-according to any one of a number ofwell-known methods, to secure greatly improved performance uniformlyover It may be shown, however,

the tuning range.

These attempts failed, however,

and many experiments have adequately verified f the fact, that thergene'..itive effect itself varies materially with frequency, even whenapplied to j a circuit of inherently uniform performance, and my presentinvention is an arrangement in which the highly desirable and advantagesresult of substantially uniform performance in a regenerative system issecired.

In my co-pending application, Serial Number 339,697, filed June 10,1940, I describe a system employing unitary rather than coupled resonantcircuits, and securing a regenerative vor negative resistance effectthrough the employment of a vacuum tube whose only function is tuning byinductance variation with a movable ferromagnetic core, however, theinductance-toresistance ratio may be maintained substantially constant,since insertion of the core provides a simultaneous increase in theinductance 'and the resistance of the resonant circuit as the frequencydecreases.

In my United States Patent No. 2,106,226, I

, disclose means for coupling two permeabilitytuned resonant circuits,each of which has a substant'ially constant ratio of inductance tohighfrequency resistance throughout the frequency range, in such a wayas to provide substantially uniform over-all amplification andselectivity. The system described in that patent is intended to operatewithout an appreciable degree o f regeneration. In accordance with mypresent invention, however, I employ a pair of permeability-tunedresonant circuits .which are so designed and so coupled as to providesubstantially luniform performance even though regeneration is utilizedto greatly improve the performance of the system.

Thus my invention provides, for the first time, means for realizing theimprovement in gain and selectivity obtainable by utilizing regenerationwithout requiring additional vacuum tubes or manual controls, andwithout sacrificing the uniformity of performance which is readilysecured by the use of movable ferromagnetic cores for tuning. While theelectrical circuit which I employ is supercially similar to earliervarrangements, it is pointed out that only by employing components whichare designed in the mannerto be disclosed herein is it possible torealize the y greatly improved performance which my system provides. Aswill be clear from what is to folto produce this effect in a controlledmanner.

In arrangements according to my present invention, on the other hand,"Iemploy coupled rather than unitary resonant circuits, and I do notemploy vacuum tubes solely to produce the regenerative eifect, each ofthe vacuum tubes in the present system acting additionally as ahighfrequency amplifier in the customary manner.

In practicing my present invention I employ one or more systems orstages each including an amplifying vacuum tube, a pair of coupledresonant circuits tuned by inductance variation with a ferromagneticcore movable relatively to the inductance coil in each circuit, meansfor producing a decrease in the effective resistance of the resonantcircuit, and an arrangement for utilizing this means in such a way thatits resistance-reducing effect varies in a desired manner with thefrequency to which the coupled resonant circuits are tuned. I thussecure a degree of gain and selectivity over' the range ofadjustabilitywhich is much greater and more uniform than could besecured by the employment of the resonant circuits alone.

If a resonant circuit is -tnned by capacitance variation, the inductanceremains fixed and the resistance increases with frequency. Thus theratio of inductance to resistance varies widely over the frequencyrange, the circuit becoming increasingly poor at the higher frequencies.By

which do not, in themselves, provide substantially uniform selectivityover the frequency range, without sacrifice of the highly desirableuniformity of performance which is a feature of the invention.

It is an object of my invention, therefore, to provide a high-frequencyamplifying system having a high degree of gain and selectivity which ismaintained substantially uniform over a wide range of frequencies.

A further object vide simple and inexpensive means for improving theselectivity and gain of a high-frequency amplifying system while at thesame time maintaining its performance substantially uniform over a widerange of frequencies.

Still another object .of my invention is to improve the over-allresonant gain and selectivity of a pair ofy inductively 'tuned resonantcircuits by a substantially uniform degree throughout a wide range offrequencies.

The attainment of these and other highlyl de- A out more fully in thespecification which follows.

The invention will now-be described by reference to the accompanyingdrawings, in which: Fig. 1 is a schematic diagram of a tunedradiofrequency amplifying stage in accordance with the invention;l

Fig, 2 is 'a graph showing the variation with frequency of some of theparameters of an embodiment of the,v arrangement shown in Fig. 1; and

Fig. 3 is a schematic diagram of acomplete radio receiver embodying theinvention.

of my inventionl is to'prol The system shown schematically in Fig. lcomprises two, resonant circuits designated generally by numerals I andII, circuit I being effectively connected across the output terminals ofa first vacuum tube I and circuit II being effectively connected acrossthe input terminals of a second vacuum tube 2. Circuit I conprisescapacitor 3 and inductor 5 with associated and relatively movableferromagnetic core 1. Circuit II comprises capacitor and inductor 6 withassociated and relatively movable ferromagnetic core 8. Cores l and Bare arranged for motion in unison relatively to inductors 5 and 6 bymeans of a suitable uni-control mechanism, as indicated by the dottedline 9. Capacitors 3 and ,4 are indicated by arrows as being adjustablefor purposes of initial alignment, but they may alternatively be madevariable and may be ganged for rotation in unison with movement of thecores 1 and 6 if desired. Capacitor I is. connected between the outputelectrodes II and I2 of vacuum tubes" I and 2 respectively.

The high-potential terminals of capacitor 3 and inductor 5 are connectedto the output electrode II of vacuum tube I. The high-potemial'terminals of capacitor 4 and inductor 6 are connected to the inputterminal I l of vacuum tube 2. The low-potential terminals of capacitors3 B+ but not shown. Circuit IIv is completed to ground through resistorI6 and capacitor I 8. Automatic volume control potential, if such isemployed, may be applied to circuit II through resistor I6. Vacuum tubesi and 2 are shown as of the pentode type, but it will be understood thatthey may alternatively be of any suitable type. v

Circuits I and II are preferably capacitively coupled, and to this endinductors 5 and 6 are either so positioned or so shielded from oneanother or both, that the inductive coupling between circuits I and IIis negligibly small. With reasonable precautions, such inductivecoupling as may remain will not be detrimental `to the properperformance of the system, and in par-V ticular circumstances, in orderto achieve a desired performance characteristic, additional amounts ofinductive coupling may be employed.

With the inductive coupling between inductors 5 and 6 negligibly small,I prefer to couple circuits I and II only through common capacitor I3and common resistor I5, which together provide a suitable variation incoupling over the frequency range, remembering that circuits I and IIare tuned by inductance variation.l In my United States Patent No.2,106,226 relating to nonregenerative systems I show various othermethods of securing suitable coupling variation between resonantcircuits inductively tuned by relatively movable ferromagnetic cores,dependv inglupon the performance characteristics` of the individualcircuits. Any of these other coupling methods might be adapted for usein connection with my present invention, but for my present` i thefurther detailed description of my system it shouldv be understood thatI have specific reference to this coupling arrangement although othersmay alternatively be employed.

Still referring to Fig. 1, in the absence of ferromagnetic cores l. and8, and if circuits I and II were tuned over any considerable range offrequencies by varying the capacitance of capacitors 3 and 4respectively, it would be impossible to introduceeven a relatively verysmall amountof regenerative amplification in one portion of thefrequency range without encountering uncontrollable self-oscillation ofthe system in some other portion of the frequency range. By means offerromagnetic cores ll and 8, however, and by the manner in which Idesign and arrange the other circuit components, as hereinafterdescribed, I am able to tune the system entirely by inductance variationand I secure su'ch control over the individual performances of circuitsI and II, and consequently over the regenerative amplification producedby vacuum tube 2, that I am enabled to greatly increase both the gainand selectivity of the system so that it equals or exceeds that whichwould otherwise be obtained only over a very limited portion of thefrequency range at one extreme thereof. I achieve 'this highlybeneficial control 'first by proper adjust-v ment of the couplingbetween circuits I and II, and second by appropriate choice ofthekferromagnetic materials employed in the cores and 8, and by suitabledesign of the cores with respect to the characteristics of inductors 5and 6 with which they are used.

The particular arrangements with regard Vt the `performance-controllingcharacteristics of cores 'I and 8 which I shall now describe are thepreferred arrangements, since they may be readily aplied in thedevelopment laboratory, and produce highly satisfactory designs. It willbeunderstood however, that within the scope of the teachings now to begiven, numerous other arrangements to secure the advantages of theinvention may be employed.

With regard to variable inductors of the type employingrelatively-movable ferromagnetic cores, such as inductors-5 and 6 withcores .'I and 8 respectively, it is known that as the core enters -thewinding it produces simultaneous increases- Vin the .effectiveinductance and in the effective It is also high-frequency resistance`thereof. known that by appropriate choice of the ferromagnetic materialemployed, these two increases may be made to have any reasonable desiredrela` tion, one to the other, so that, for example, a resonant circuitemploying the device may be designed to have av substantially constantratio of inductance to resistance (L/R) over the' frequency range.

It is also known that the regenerative effect produced by anappropriately connected vacuum tube varies between wide limits over thefrequency range, thatthe specific manner of this variation ,will dependin each case upon the specific circuit `arrangement employed, and that,in general, the .variation of the regenerative effect will be markedlydifferent from the variation in the performance of the associatedresonant circuits per se. In accordance with the invention. I overcomethese difficulties by employing two circuits cuit tol lower frequencies.

It will be apparent that in any specific embodiment, any core chosen,for example, for use with inductor in circuit I, will produce a definiteand measurable variation (or constancy) of the performance of circuit Iper se over the frequency range. In general, it is possible to thendesign a. core for use with inductor 6 in circuit II which willcompensate for (1) the variation in circuit I,

(2) the variation in the effective coupling be' tween circuits I and II,(3) lthe variation in the regenerative effect of vacuum tube 2, and (4)the otherwise uncontrolled variation in the performance of circuit II.Similarly, ifa core be chosen for circuit II, a core may then bedesigned for circuit I which will bring about similar compensations. Thecompensations thus achieved may be such as'to produce substantialuniformity of performance over the frequency range, or, if desired forspecial purposes, may' be such as tor produce some particular desiredvariation of performance with frequency other than that which wouldresult without the control provided by the cores.

Assuming that it is desired toachieve substantially constant performanceover the frequency range,the two cores 1 and 8 may be made identical,and under these circumstances will require each to haveresistance-producing losses somewhat less than those required to producesubstantially constant L/R in either of the circuits separately.Similarly, a core having losses higher than those -necessary to produceconstant L/R in either of the circuits may be employed in one of them,but in this direction lies a limit beyond which it will be foundimpossible to secure adequate compensation by designing the core for theothercircuit to have relatively very low losses.

Various combinations of lcore characteristics may be found, however,which while having adequate effective .permeability to tune over thedesired -frequency range, will co-act to achieve substantial constancy,or any reasonable desired varia.

tion, of the performance of the system. There-'- fore, while I shall nowdescribe a preferred arrangement according to the invention, variousother possible combinations such as those mentioned above, lie withinthe scope thereof.

I prefer to so construct the core 8 with respect to the material ofwhich it is made and its proportions relative to the winding of inductor6 that resonant circuit II, measured by itself, will havea-substantially constant ratio of inductance to resistance over thefrequency range.

Since the inductance of the circuit will be inversely proportional tothe square of the frequency, constant L/Rl requires that the resistancebe also inversely proportional to the square of the frequency, with aslight additional resistance increase as thefrequency is lowered tocompensistance I5 to the impedance of circuit II, and l sate for thedecrease in the resistance of the@ 66 in circuit I is materiallyimproved as the core 1 is inserted into the inductor 5 to tune the cir-The most advantageous law of variation of the L/R ratio in circuit I isnot a simple one. In a system tunable from 550 to 1625A kilocycles, forexample, the ratio was inversely proportional to approximately the 0.5power of the frequency as the core first entered the inductor at thehigh-frequency end of the tuning range, and gradually increased toinverse proportionality to approximately the .with frequency in a systemaccording to Fig. 1

and having cores of the types just described. As will be seen, circuitII has nearly constant L/R and circuit I has L/R increasing sharply withdecreasing frequency. The graph also shows curves of selectivity or bandwidth in kilocycles and sensitivity in microvolts per meter, of a radioreceiver embodying the above described tuning system. It will be notedthat as to both sensitivity and selectivity the performance of thereceiver was remarkably constant over the frequency range.

An empiric equation for the curve of L/R. in circuit I as shown in Fig.2 will have the form f=G(L/RD)u 1) where j is the frequency and D and Gare constants. preferred arrangement, with core 8 (Fig. 1) 'giving verynearly constant L/RI in circuit II, the constants for the above equationwere D=4.2 and G=1202 and the exponent `was 0.334. This equationreproduced the measured L/R curve of circuit I as shown in Fig. 2 towithin 5.5% at all points.

By analysis of the equations of the electrically equivalent circuit, andassuming that the induct where K is the ratiol of the mutual impedanceconsisting principally of capacitance 'I3 and reinl which the numericalsubscripts correspond to the numerals in Figs. 1 and 3, the presence ofregeneration being indicated by the plus signs in Equations (2) and (3)and the minus sign in Equation 4.' f

At resonance the inductive reactance of the system is equal to itscapacitive reactance. The time constant (L/R) and the dynamic resistance(L/RC) therefore are y && JL3 L5 1+K2 2 RC' R R Cadts-KZRQ) where w=21rfand f is the frequency.

Since the circuit is tuned by inductance variation, L5 is inverselyproportional to' the square of thefrequency. The Values of K, Rs and Re,are controllable through the design of cores 1 and 8. It is apparentfromEquations 5 and 6 therefore, that either constant selectivity (L/R)or constant gain (L/RC) may be achieved, or that a compromise betweenthem may be obtained as lthe designer may desire. In the preferredembodiment about to be described, the performance of which is indicatedin Fig. 2, such a compromise has been secured.

For a particular embodiment -in the y Wire On a. .200" 'I. D. by .224"O. D. tube, the

core, and comprises hydrogen-reduced iron dust and instructions forconstructing a pair of cores Which will perform in accordance with thepreierredarrangement above described, it being understood that theseconstants aregiven merely by way of illustrative example and that theyare therefore not to be taken as in anyway limiting the' scope of theinvention which is defined in the appended claims.

In Fig. 3, antenna i9 is connected to variable inductor 20and capacitors2| and 22 to form a tuned input circuit connected to the control grid 23Ofvacuum tube 24, which is preferably a triple-grid super-controlamplifier of the type known to the trade as the 6K7. Vacuum tube 25 ispreferably a triple-grid detector amplier tube of the type known to thetrade as the 6.17 vand is arranged as shown to function as a detectorand to supply audio-frequency voltage to vacuum tube 26, which may be abeam power amplifier of the type known to the trade as the25L6, and

which supplies audio-frequency current to loud 25 speaker 21. Plate andscreen-grid voltages are supplied to vacuum'tubes 24. 25 and 26 througha i'llter comprising choke ,29 and capacitors 30,

' 3| by rectifier Vacuum tube 28, which may be of the type knownto thetrade as the 25Z5, and 30 which is supplied with 60-cycle 11G-voltenergy from the lighting circuit. The source of heater or filamentvoltages for vacuum tubes. 24, 25, 26 and 21 is not shown but may bearranged in the conventional manner. The regenerative coupled-circuitsystem of Fig. 1 is connected as shown between vacuum tubes 24 and 25,ferromagnetic cores 1, 8 'and 32 being ganged together throughA asuitable uni-control mechanism for operation in unison to tune theentire receiver 40 to a desired signal. y In an' illustrative embodimentaccording to Fig. 3 thecoils 5 and 6 maybe alike and may each have 207'turns of No. 37 single enamelled winding length ,being 1.125". Thesecoils will have an inductance of approximately 47 ph. and will have aquality coeiicient Q of at 1625,' kilocycles when properly made. Asuitable arrangement for coils 5', 6 and 20 is to place them side byside with their axes parallel and approximately 1%." apart, with platecoil 5 in the center and with a grounded'conductive shield f aroundit,no other shielding of the coils b eing necessary.

drogen-reduced `iron dust sifted through a 300- mesh screen,insulatedwith a suitable insulating varnish and molded withirml 2 to 3percent of powdered Bakelite at Vrelatively low pressure.`

Core 8, onthe other hand,- is not homogeneousf but may consist of twodifferent portions. The portion which last enters the coil occupiesapproximately '10 percent of the totallengthof the sifted through al1GO-mesh screen. The remaining 30 percent of the length `of the core mayconsist of approximately 80 percent of 30D-mesh hydrogen iron dust and20 percent of iron reduced from carbonyl of the grade known to the trade70 Vhtf' long. Core 1 will weigh approximatelyi 75 y 'grams' and have adensity of about 5.8." Core 8 `will weigh about 3.9 grams and willhaves. density of approximately 5.65.

' The remaining lcomponents in an embodiment in accordance with Fig. 3may be as follows:

It will be understood that depending upon the performancecharacteristics of the particular tubes employed, the regenerativecoupling ca-` pacitor l0 is to be adjusted to provide the requiredsensitivity. By proper development and choice of the constants, inaccordance with the instructions herein given, the system is entirelystable in operation, being well away from the condition of incipientoscillations 'at all -frequencies, but will at the same time providehigh selectivity.

I prefer to secure 'control of the performance of my system by employinga coil 6 and corev 8 in circuit II, which in themselves producesubstantially constant L/R and by employing a coil 5 and core 1 incircuit I, which produce an L/l'iI rapidly decreasing with increasingfrequency, as

illustrated in Eig. 2. It will be understood, however, that control mayalso be achieved by employing a constant L/R coil and core combinationin circuit I and by providing a compensating L/R characteristicin-circuit II. It is also to be understood that the characteristiccurves of Fig.`

2 are those of a practical embodiment of the invention and do notrepresent the best that may be achieved, either from -the standpoint ofconstancy of gain or the standpoint of constancy of selectivity, andthat when, in particularcases;`

variation of either the selectivity or the fsens'iif tlvity is desiredover the frequency band,'this` may also 'be secured without departing 1from the scope of the invention.

From the above it appears that by the regenerative effect produced andcontrolled asabove described, cooperating with the compensations andcircuit characteristics described, I am able to maintain the effectivehigh-frequency resistance of the .amplifier at a value only slightlygreater than zero throughout the 4tunable range of frequencies of theamplifier.

Having thus described my invention, what I claim is:

1'. A tunable signal-frequency selective ampli- 1 55 iler for use inradio receivers and the like in- Y Core 1 may be homogeneous and made ofhy-` cluding first and second resonant circuits veach having aninductive winding, a non-inductive couplingrimpedance common to saidcircuits, a.

vacuum tube having its input terminals coni nected across said secondcircuit and having an output electrode connected through a capacitanceto the high-potential side of said iirst lcircuit, a ferromagnetic coremovable relatively to the winding in said first circuit and having suchcharacteristics as to cause the inductance-'to-re. sistanceratio in saidrst circuit to decrease with increasing frequency, and a ferromagneticcore movable relatively to the winding in said sec' ond circuit andhaving such characteristics Ias to maintain the inductance-to-resistanceratio in said secondA circuit substantially constant, said ,v coresbeing movable in unison to tune said amplifier over a range offrequencies.

2. AA tunable signal-frequency selective ampliner for use in radioreceivers and the like includan adjustable capacitor and an inductiveWinding, a coupling impedance comprising a resistor and a capacitorconnected in parallel betweenthe low-potential terminals of saidadjustable capacitors and ground, a vacuum tube having its inputterminals connected across said second circuit and having an outputelectrode connected through a capacitance to the high-potential side ofsaid first circuit, a ferromagnetic core movable relatively to theWinding in said first circuit and f having such characteristics as tocause the inductance-to-resistance ratio in said first circuit todecrease with increasing frequency, and a ferromagnetic core movablerelatively to the winding in said second circuit and having suchcharacteristics as to maintain the inductance-to-resistance ratio insaid second circuit substantially constant, said cores being. movable inunison to tune said amplifier over a range of frequencies.

3. A tunable signal-frequency selective amplifier for use in radioreceivers and the like including first and second resonant circuits eachhaving an vinductive winding, a non-inductive coupling impedance commonto said circuits, a vacuum tube having its input terminals connectedacross said second circuit and having an output electrode connectedthrough a capacitance to the high-potential side` of said first circuit,a ferromagnetic core movable relatively to the winding in one of saidcircuits and having such characteristics as to cause theinductance-toresistance ratio in said one circuit to decrease `withincreasing frequency, and a ferromagnetic core movable relatively to thewinding in the other of said circuits and having such characteristics asto maintain the inductance-to-resistance ratio in said other circuitsubstantially constant, said cores being movable in unison to tune saidamplifier over a range of frequencies. A

4. A 'tunable signal-frequency selective amplifier for use in radioreceivers and the like including rst and second resonant circuits eachhaving an adjustable capacitor and an inductive winding, a couplingimpedance comprising a resistorand a capacitor connected in parallelbetween the low-potential terminals of said adjustable capacitors andground, a vacuum tube having its input terminals connected across saidsecond circuit and having an output electrode connected through acapacitance to the high-pctential side of said rst circuit, aferromagnetic core movable relatively to the winding in one of saidcircuits and having such characteristics as to cause theinductance-to-resistan-ce ratioin said one circuit to decreasewithincreasing frequency, and a ferromagnetic core movable relatively tothe winding in the other of said circuits and having suchcharacteristics as to maintain the inductance-to-resistance ratio insaid other circuitsubstantially constant, said cores being movable inunison to tune said amplifier over a range of frequencies.

`5.,A tunable signal-frequency selective amplifier for use in radioreceivers and the -like including first and second resonant circuitseach having an inductive winding, a non-inductive coupling impedancecommon to said circuits, a vacuum tube having its input terminalsconnected across Ysaid second circuit and having an output electrodeconnected through a. capacitance to the high-potential side of saidfirst` circuit, and a ferromagnetic core movable relatively to theinductive winding in each of said circuits, said cores being movable inunison to tune said amplifier over a range of frequencies, saidnoninductive coupling impedance having a rising coupling effect withdecreasing frequency, and the impedance lof said output electrodeconnection increasing with decreasing frequency, said ferromagneticcores having such respective characteristics as to compensate for saidrising coupling effect and said increasing impedance effect to establisha substantially constant selectivity for said amplifier throughout thetuning range.

`6. A tunable signal-frequency selective amplifier for use in radioreceivers and the like including 'first and second resonant circuitseach having an inductive winding, a non-inductive coupling impedancecommon to said circuits, a vacuum tube` having its input nterminalsconnected across said second circuit and having an youtput electrodeconnected through a capacitance to the high-potential side of said firstcircuit,

and a ferromagneticcore movable relatively to I inductive couplingimpedance having a rising coupling effect with decreasing frequency, andthe impedance of said output electrode connection increasing withdecreasing frequency, said ferromagnetic cores having respectivecharacteristics producing inductance-to-resistance ratios in saidcircuits compensating for said rising coupling effect and saidincreasing impedance effect to establish -a substantially constantselectivity for said amplifier throughout the tuning range.

7. A tunable signal-frequency selective amplifier for use in radioreceivers and the like including first and second resonant circuits eachhaving an inductive winding, a non-inductive coupling impedance commonto said circuits, a vacuum tube having its input terminals connectedacross said second circuit and having an output electrode connectedthrough a capacitance to the high-'potential side of said first circuit,and a ferromagnetic core movable relatively to the inductive winding ineach of said circuits, said cores being movable in unison to tune saidamplifier over a range of frequencies, said cores being free from commonelectrical connections' and producing independent effects upon saidcircuits and having characteristics coacting with said couplingimpedance and with said output electrode connection to establish asubstantially constant selectivity for said amplifier throughout thetuning range.

8. A tunable signal-frequency selective amplifier for use in radioreceivers and the like iny circuits, said cores being movable in unisonto tune said amplifier over a range of frequencies, said non-inductivecoupling impedance having a rising coupling effect. with decreasingfrequency,

- and the impedance of said output electrode connection increasing withdecreasing frequency, said ferromagnetic cores having such respective.characteristics `as to compensate for said rising coupling effect andsaid increasing impedance effect to establish a substantially constantselectivity for said amplier throughout the tuning range, said coresbeing free from common electrical connections and producing independenteffects upon said circuits.

9. A tunable signal-frequency selective ampliiler for use in radioreceivers and the like in cluding first andvsecond resonant circuitseach having an inductive winding, a non-inductive coupling impedancecommon to said circuits, a 10 vacuum tube having its input terminalsconnected across said second circuit and having an output electrodeconnected through a capacitance to lthe high-potential side `of said rstcircuit, and

